1. Field of the Invention
The invention relates to a metal complex and to a method of making the same. The invention also relates the use of the metal complex, to a light emitting device including the metal complex, and to a method of making the light emitting device.
2. Related Technology
Much effort has been devoted to the improvement of the emission efficiency of light-emitting devices (LEDs) either by developing highly efficient materials or efficient device structures.
FIG. 1 shows cross-section of a typical LED. The device has an anode 2, a cathode 5 and a light emissive layer 4 located between the anode and the cathode. The anode may be, for example, a layer of transparent indium tin oxide. The cathode may be, for example, LiAl. Holes and electrons that are injected into the device recombine radiatively in the light emissive layer. A further feature of the device is the optional hole transport layer 3. The hole transport layer may be a layer of polyethylene dioxythiophene (PEDOT) for example. This provides an energy level which helps the holes injected from the anode to reach the light emissive layer.
Known LED structures also may have an electron transport layer situated between the cathode 5 and a light emissive layer 4. This provides an energy level which helps the electrons ejected from the cathode to reach the light emissive layer.
In an LED, the electrons and holes that are injected from the opposite electrodes are combined to form two types of exitons; spin-symmetric triplets and spin-anti-symmetric singlets. Radiative decay from the singlet (fluorescence) is fast, but from the triplet (phosphorescence) is formally forbidden by the requirement of the spin conservation.
Many have studied the incorporation of phosphorescent materials into the light emissive layer by blending. Often, the phosphorescent material is a metal complex, however it is not so limited. Further, metal complexes also sometimes are fluorescent.
A metal complex will comprise a metal ion surrounded by ligands. A ligand in a metal complex can have several roles. The ligand can be an “emissive” ligand which accepts electrons from the metal and then emits light. Alternatively, the ligand may be present simply in order to influence the energy levels of the metal. For example, where emission is from a ligand, it is advantageous to have strong field ligands coordinated to the metal to prevent energy loss via non-radiative decay pathways. Common strong field ligands are known to those skilled in this art and include CO, PPh3, and ligands where a negatively charged carbon atom bonds to the metal. N-donor ligands are also strong field ligands, although less so than those previously mentioned.
The effect of strong field ligands can be appreciated from an understanding of the mechanism by which light is emitted from a metal complex. Three reviews of luminescent metal complexes that provide an appreciation of this mechanism are referred to below.
Chem. Rev., 1987, 87, 711-7434 is concerned with the luminescence properties of organometallic complexes. This review paper provides a brief summary of the excited states commonly found in organometallic complexes. The excited states that are discussed include metal-to-ligand charge-transfer (MLCT) states, which involve electronic transitions from a metal-centered orbital to a ligand-localized orbital. Thus, in a formal sense this excitation results in metal oxidation and ligand reduction. These transitions are commonly observed in organometallic complexes because of the low-valent nature of the metal centre and the low-energy position of the acceptor orbitals in many ligands. Ligand to metal charge-transfer (LMCT) states also are mentioned which involve electronic transitions from a ligand-localized orbital to a metal-centered orbital.
In the section of the article that deals with photoluminescence, a sub-section is dedicated to metal carbonyl complexes, which are said to be recognized as being some of the most light-sensitive inorganic materials. Examples include M(CO)6− (M=V, Nb, Ta); and M(CO)6(M=Cr, Mo, W).
Matrix isolation studies of M(CO)5L complexes, where M=Mo or W and L=pyridine, 3-bromopyridine, pyridazine, piperidine, trimethylphosphine, or trichlorophosphine, are reported also as they are said to have provided the first reports of fluorescence from substituted metal carbonyls.
Several Mo(CO)5L complexes, where L=a substituted pyridine ligand, are also mentioned and it is said that they are known to luminesce under fluid conditions. The emission has been assigned to a low-lying MLCT excited state.
Other sub-sections in this review article are dedicated to dinitrogen complexes; metallocenes; metal isocyanides; alkenes; and ortho-metallated complexes.
It is said that a number of examples of ortho-metallated complexes have been shown to luminesce in room temperature solutions. For example, the emission spectrum of [Ru(bpy)2(NPP)]+ is said to exhibit the structure associated with MLCT emission. Several ortho-metallated Pt(II) complexes also are mentioned where it is said that the emission may be assigned to a MLCT excited state.
The review article summarizes that low-lying MLCT excited states are often observed, because of the low-valent metal centres and vacant low-energy ligand acceptor orbitals in organometallic complexes. Further, it is reported that relationships exist between the energy ordering of the excited-state levels and the observed photophysical and photochemical properties. Still further, it is said that the great majority of examples of room temperature emission have been attributed to MLCT excited states.
Analytical Chemistry, Vol. 63, No, 17, Sep. 1, 1991, 829A to 837A is concerned with the design and applications of highly luminescent transition metal complexes especially those with platinum metals (Ru, Os, Re, Rh and Ir).
Table I in this paper lists representative metal complexes categorized by luminescence efficiency. The systems are limited to those containing at least one α-diimine ligand such as 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen), although many of the design rules and fundamental principles are said to apply to other classes of luminescent metal complexes.
In this paper it is explained that transition metal complexes are characterized by partially filled d orbitals and that to a considerable extent the ordering and occupancy of these orbitals determine emissive properties.
For a representative octahedral MX6 d6 metal complex, where M is the metal and X is a ligand that coordinates at one site, it is explained that the octahedral crystal field of the ligands splits the five degenerate d orbitals into a triply degenerate t level and a doubly degenerate e level. The magnitude of the splitting is given by the crystal field splitting, which is a particularly important parameter for determining the luminescence properties of the complex, whose size is determined by the crystal field strength of the ligands and the central metal ion. The luminescence properties of the complex thus can be controlled by altering the ligand, geometry, and metal ion.
There are three types of excited states mentioned in this paper: metal-centred d-d states, ligand-based π-π* states, and charge-transfer states.
Charge-transfer (CT) states involve both the organic ligand and the metal. As mentioned above, metal-to-ligand charge transfer (MLCT) involves promoting an electron from a metal orbital to a ligand orbital and ligand-to-metal charge transfer (LMCT) involves promoting an electron from a ligand to a metal orbital.
According to this paper, the most important design rule of luminescent transition metal complexes is that the emission always arises from the lowest excited state. Thus control of the luminescence properties of complexes hinges on control of the relative state energies and the nature and energy of the lowest excited state. In this regard, the paper states that any metal-centred d-d states must be well above the emitting level to prevent their thermal excitation, which would result in photochemical instability and rapid excited-state decay. Therefore, one of the more important criteria is to remove the lowest d-d state from competition with the emitting level. Thus a desirable design goal is to make the d-d state as thermally inaccessible as possible from the emitting MLCT or π-π* state. Controlling the energies of the d-d states is accomplished by varying either the ligands or the central metal ion to affect the crystal field splitting. Stronger crystal field strength ligands or metals raise d-d state energies, and crystal field strength increases in the series
Cl <py <<bpy, phen <CN <CO
where py represents pyridine.
For a metal, the crystal field splitting increases when descending a column in the periodic table. CT state energies are affected by the ease of oxidation/reduction of the ligands and metal ion. For MLCT transitions, more easily reduced ligands and more easily oxidated metals lower the MLCT states.
The π-π* state energies are largely dictated by the ligands themselves. However, the energies and intensities of the π-π* transitions can be altered by varying either the substituents, the heteroatoms in the aromatic ring, or the extent of π conjugation.
Photochemistry and Luminescence of Cyclometallated Complexes, Advances in Photochemistry, Volume 17, 1992, page 1 to 68 describes that most of the attention in this field has been focused on complexes of the polypyridine-type family (prototype: Ru(bpy)2+3, where bpy=2,2′ bipyridine).
The interest in the photochemical and photophysical properties of cyclometallated complexes is said to be an extension of this.
Table 2 in this publication shows absorption and emission properties of cyclometallated ruthenium, rhodium, iridium, palladium and platinum complexes and their ligands.
Metal complexes containing a PPh3 strong field ligand are disclosed, for example in JP 2002-173674, which discloses a rhenium complex as shown below.

Among others, tungsten, osmium, nickel and platinum complexes with PPh3 ligands of the type shown below are known.

For example, see Journal of the Chemical Society, Dalton Transactions: Inorganic Chemistry (1972-1999) (1986), (3), 511-15 (tungsten) and Zeitschrift fur Anorganische und Allgemeine Chemie (1975), 418(1), 45-53 (nickel).
Also, the following osmium complex is known from Adv. Mat. 2002, 14, 433.

The following complex is disclosed in EP 1353388:

and the following complex is disclosed in JP 2002-203678:

The PPh3 ligands in these complexes are not emissive ligands.
Further complexes which are comprised of two different types of reactive metal centre linked by a di-topic ligand are disclosed in J. Chem. Soc. Dalton Trans. 2002, 2423-2436. It is said in this paper that triphenylphosphine is one of the more commonly used ligands in organic chemistry and is one of the most common ligands found in homogenous transition metal catalysts. As such, this paper is not concerned specifically with the field of light emitting devices but rather with the use of metal complexes as catalysts. Various PPh3 complexes are mentioned.
One problem associated with metal complexes including those with a PPh3 ligand relates to aggregation, which leads to triplet quenching and also reduces the efficiency of exiton transfer. Further, many metal complexes with a PPh3 ligand are not solution processable. This creates problems during manufacture of a light emitting device where it is advantageous to be able to cast the layer of the device by solution processing. A further problem relates to phase separation of the metal complex and the host material, where a separate host material is used.
In view of the above, it will be appreciated that there is a need to improve known metal complexes, many of which contain a PPh3 ligand, for use in light emitting devices.